The ratio of the reactants and products of gases can be expressed in simple ratio.
This Law is Given By Gay Lussac.

For Example:- 2C + O2 --> 2CO2
Here Volume of,
Carbon Atoms: 2
Oxygen Atoms: 1
Carbon Dioxide Molecule: 2
So the ratio will be 2:1:2 

And we can see that it is a simple ratio!

**This is only applicable for gases!